To enable the transition away from a fossil-fuel based energy economy, it is essential to address the problem of energy storage in addition to that of energy extraction from sustainable resources. In particular, if one is to utilize solar energy, a resource that is available in large excess relative to current consumption rates, the photon energy must be stored and made available for use after dark. Several possibilities are already being pursued in laboratories across the world including high energy density batteries, hydrogen production via electrolysis, and hydrogen production via direct photolysis. Despite these efforts, large-scale energy storage technologies remain elusive.
Due to the attractiveness of chemical fuels for energy storage and the relative ease with which heat can be extracted from other inputs, thermochemical production of hydrogen has received significant, though sporadic, interest over the past few decades (Funk, J. E. Int. J. Hydrogen Energy 26 185 (2001)). The most aggressively pursued reaction cycles at present are those involving (1) the high temperature dissociation of ZnO(s) to Zn(g) and O2(g) and (2) the partial reduction of Fe3O4(s) to FeO(s) and O2(g) or analogously NiFe2O4(s) to (Ni1/3Fe2/3)O(s) and O2(g) (Kodama, T. & Gokon, N. Chem. Rev. 107, 4048-4077 (2007); Perkins, C. & Weimer, A. Int. J. Hydrogen Energy 29, 1587-1599 (2004); Steinfeld, A. Solar Energy 78, 603-615 (2005); Tamaura, Y., Steinfeld, A., Kuhn, P. & Ehrensberger, K. Energy 20, 325-330 (1995)). While development activities are ongoing in both classes, several fundamental challenges remain. In particular, key obstacles are connected to the structural transition (or change in phase) that these materials undergo in transforming between oxidized and reduced forms. For example, on reduction, solid ZnO transforms to gaseous Zn, whereas Fe3O4 with the spinel crystal structure transforms to FeO with the rock salt crystal structure. Such phase changes ultimately cause morphological changes that slow the kinetics of oxygen uptake and release and hence degrade fuel productivity with cycling. A second challenge with these thermochemical cycles is the absence of any demonstration to date of CO2 reduction to produce conventional, carbon-containing fuels.
What is needed then is a material that can prepare fuels without undergoing performance degradation. Preliminary demonstrations of hydrogen production from CeO2 have appeared in the literature in the past two years (Abanades, S. & Flamant, G. Sol. Energy 80, 1611 (2006); Kaneko, H. et al. J. Mater. Sci. 43, 3153-3161 (2008); Kaneko, H. et al. Energy Fuels 21, 2287-2293 (2007); Kaneko, H. et al. Energy 32, 656-663 (2007); Kang, K.-S., Kim, C.-H., Park, C.-S. & Kim, J.-W. J. Ind. Eng. Chem. 13, 657-663 (2007); Miller, J. E. et al. J. Mater. Sci. 43, 4714-4728 (2008)). The present invention shows that, surprisingly, ceria can be used to prepare carbon containing fuels and, because oxygen uptake and release in ceria does not induce a structural change, ceria based materials can be cycled without a degradation in fuel productivity.